Chromogranin A knockout (Chga-KO) mice display increased adiposity despite high levels of circulating catecholamines and leptin. Consistent with diet-induced obese (DIO) mice, desensitization of leptin receptors (Ob-R) due to hyperleptinemia is believed to contribute to the obese phenotype of these KO mice. In contrast, obesity in Ob/Ob mice is caused by leptin deficiency. To characterize the metabolic phenotype, Chga-KO mice were treated with a CHGA-derived peptide catestatin (CHGA352-372) that is deficient in these mice. CST treatment reduced fat depot size and increased lipolysis and fatty acid oxidation. In liver, CST enhanced oxidation of fatty acids as well as their assimilation into lipids, effects that are attributable to the upregulation of genes promoting fatty acid oxidation (Pparα, Acox, Ucp2 and Cpt1α) and incorporation into lipids (Gpat and Cd36). CST did not affect basal or isoproterenol-stimulated cAMP production in adipocytes but inhibited phospholipase-C activation by the alpha-adrenergic receptor (α-ADR) agonist phenylephrine suggesting inhibition of α-ADR signaling by CST. Indeed, CST mimicked the lipolytic effect of the α-ADR blocker phentolamine on adipocytes. Moreover, CST reversed the hyperleptinemia of Chga-KO mice and improved leptin signaling as determined by phosphorylation of AMPK and Stat3. CST also improved peripheral leptin sensitivity in DIO mice. In Ob/Ob mice, CST enhanced leptin-induced signaling in adipose tissue. In conclusion, our results implicate CST in a novel pathway that promotes lipolysis and fatty acid oxidation by blocking α-ADR signaling as well as by enhancing leptin receptor signaling.
Chromogranin A (CHGA in humans, Chga in mice), a 48-kDa acidic secretory proprotein (1-3), gives rise to several peptides of biological importance, which include the dysglycemic hormone pancreastatin (PST: CHGA250-301) (4,5), the vasodilator vasostatin (CHGA1-76) (6), and the anti-hypertensive peptide catestatin (CST: CHGA352-372) that inhibits nicotine-induced catecholamine release (7-9). Initially identified as a physiological brake in catecholamine secretion (7), CST has been established as a pleiotropic hormone having effects on promoting angiogenesis (10), lowering of blood pressure (8,11,12) and cardiac contractility (13-15) as well as enhancing baroreflex sensitivity (16,17) and heart rate variability (18).
In addition to the above cardiovascular functions, CST has an antimicrobial activity (19,20) and also regulates mast cell migration, cytokine production and release (21), smooth muscle cell proliferation (22), and monocyte migration (23). CST can act both extracellularly and intracellularly because the peptide can cross cell membrane (24,25).
Fat cell functions are regulated by catecholamines through four types of adrenergic receptors (ADR): β1, β2, β3 and α2 (26,27). Activation of the three β-ADRs is positively coupled to adenylyl cyclase by stimulatory GTP sensitive proteins, resulting in enhanced production of cyclic AMP. Cyclic AMP activates protein kinase A (PKA), which in turn phosphorylates hormone sensitive lipase (HSL) leading to hydrolysis of triglycerides (lipolysis). In contrast, α2-ADR activation has the opposite effects on lipolysis because it is coupled to inhibitory GTP sensitive proteins (28-31). Therefore, the net action of catecholamines depends on the balance between β- and α-ADRs (27). Normally, the β-ADR-mediated lipolytic action dominates. Sustained activation of sympathetic nervous system or increased plasma catecholamines is often associated with desensitization of β-ADR (32). In vivo studies have shown that the lipolytic action of catecholamines is blunted in obese subjects (33,34). Catecholamine-induced regulation of lipolysis through β-ADR desensitization has also been demonstrated in vitro (32,35). Repeated treatment with epinephrine results in the suppression of basal and epinephrine-stimulated lipolysis in lean and obese subjects (36). Even the in vivo lipolytic response to epinephrine is desensitized by prior exposure to epinephrine (37). In view of the above, we hypothesize that the increased fat mass in hyperadrenergic Chga-KO mice (38) reflects β-ADR desensitization by increased plasma catecholamines (8). Since catecholamines are known to inhibit leptin secretion (39-41), β-ADR desensitization may prevent such inhibition and lead to increased leptin level along with the increased adipose mass as found in Chga-KO mice and other obese models. Chronic hyperleptinemia in turn may desensitize Ob-R and perpetuate the obese phenotype.
The invention, in one embodiment, is based on CST that breaks this cycle and reduces obesity by restoring ADR and Ob-R sensitivity through normalization of catecholamine and leptin levels. Indeed, we found that chronic CST administration to obese Chga-KO mice resulted in a dramatic lean phenotype. CST treatment also reduced body weight and adipose mass in DIO mice without reducing food intake. Interestingly, CST could enhance leptin effects on adipose tissue metabolism and signaling in both DIO and leptin-deficient Ob/Ob mice. Our findings suggest that the reduction in fat mass after chronic CST treatment is due to increased lipolysis and lipid mobilization through CST action on α2-ADR and leptin receptor. In line with this, CST promoted fatty acid oxidation and leptin signaling.